<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>BaiDu</title>
    <style>
        #a {
            color: #50afeb;
        }
        #b{

        }
    </style>
</head>
<body>
<div>
    <P>
        <h1>
            <a href="https://baidu.com" id="a" onmouseover=" overFunction()" onmouseout="a()">百度</a><br>
            <span id="b" style="display: none">百度一下你就知道</span>
        </h1>
    </P>
</div>
<script>
    function overFunction(){
        document.getElementById("b").style.display="block";
    }
    function a() {
        document.getElementById('b').style='display: none';
    }

</script>
</body>
</html>